toke.c: Remove unnecessary assignment

LOP returns, so the value of orig_keyword is not used after this.
perldiag: Remove ‘The %s feature...’

The idea when this was added was for ‘use feature’ to warn when
an experimental feature was enabled.  Things didn’t turn out that
way.  Only lexical subs follow that wording, and they have their
own entry in perldiag, so this one is unnecessary.